This invention relates to communication systems having synchronized transmitters and receivers and more particularly to indicating to a transmitter loss of synchronization with a receiver and even more particularly to transmitters and receivers in radiotelephone systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS standard. This application focusses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques, with pseudo-noise scrambling codes and orthogonal channelization codes separating base stations and physical channels (terminals or users), respectively, in the downlink (base-to-terminal) direction. Since all users share the same radio resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by a transmit power control (TPC) mechanism, in which, among other things, base stations send TPC commands to users. The TPC commands cause the users to increase or decrease their transmitted power levels by increments, thereby maintaining target signal-to-interference ratios (SIRs) for the dedicated physical channels (DPCHs) between the base stations and the users. WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a mobile radio cellular telecommunication system 10, which may be, for example, a CDMA or a WCDMA communication system. Radio network controllers (RNCs) 12, 14 control various radio network functions including for example radio access bearer setup, diversity handover, etc. More generally, each RNC directs mobile station (MS), or remote terminal, calls via the appropriate base station(s) (BSs), which communicate with each other through downlink (i.e., base-to-mobile or forward) and uplink (i.e., mobile-to-base or reverse) channels. RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26. Each BS serves a geographical area that can be divided into one or more cell(s). BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26. The BSs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Both RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes like a mobile switching center (not shown) and/or a packet radio service node (not shown).
As user terminals move with respect to the base stations, and possibly vice versa, on-going connections are maintained through a process of hand-off or handover. For example in a cellular telephone system, as a user moves from one cell to another, the user's connection is handed over from one base station to another. Early communication systems used hard handovers, in which a first cell's base station (covering the cell that the user was leaving) would stop communicating with the user just as the second base station (covering the cell that the user was entering) started communication. Modern systems typically use soft handovers, in which a user is connected simultaneously to two or more base stations. In FIG. 1, MSs 28, 30 are shown communicating with plural base stations in diversity handover situations. MS 28 communicates with BSs 16, 18, 20, and MS 30 communicates with BSs 20, 22. A control link between RNCs 12, 14 permits diversity communications to/from MS 30 via BSs 20, 22.
During soft handovers (SHOs), terminals receive TPC commands from more than one base station, and methods have been developed for handling conflicts between TPC commands from different base stations. Conflicts are expected because as the user terminal or equipment (UE) leaves one cell, that cell's base station receives a progressively weaker signal and thus that base station's TPC commands call for more power, and at the same time, the user terminal may be entering a new cell, and the new cell's base station receives a progressively stronger signal and thus the new base station's TPC commands call for less power. In a 3GPP-compliant system, the UE combines TPC commands from reliable downlinks with a logical OR function, which leads to reduced UE transmit power if any of the reliable commands says “DOWN”. This is described in Section 5.1.2.2.2.3 of 3GPP TS 25.214 (V5.6.0) Rel. 5 (2003), Physical layer procedures (FDD).
Reliable “OR” TPC combining can be implemented in different ways in a UE, for instance by using reliability thresholds, which are described in N. Wiberg, H. Rong, F. Gunnarsson, and B. Lindoff, “Combining of power control commands during soft handover in WCDMA”, Proc. of the 14th Int'l Symposium on Personal, Indoor and Mobile Radio Communication (PIMRC), 2003. Other aspects of TPC are described in U.S. Pat. No. 6,594,499 to A. Andersson et al. for “Downlink Power Control in a Cellular Telecommunications Network”.
Soft handover in WCDMA and other 3G communication systems involves an Active Set Update-ADD procedure that is described, for example, at 3GPP TS 25.214 cited above. The UE reports event 1A (Radio Link Addition) to the network and the RNC informs the new base station, node B, to start uplink (UL) synchronization. When an acknowledgement message from the node B is received in the RNC, an “Active Set Update—ADD” message is transmitted to the UE, and simultaneously, the new node B starts to transmit on the downlink (DL). Until UL synchronization is achieved, the TPC commands transmitted by node B on the new DL call for the UE to increase its transmitted power; according to Section 5.1.2.2.1.2 of TS 25.214, the TPC command sequence is . . . 11111 . . . . The UE receives and decodes the “Active Set Update-ADD” message, and after that the terminal's physical layer starts to combine the DL information, including TPC commands, from node B and the “old” base station, node A.
UL synchronization when entering or adding a link in soft handover can take 100 milliseconds (ms) or even more, depending on channel conditions. This delay is mainly due to node B's having no knowledge of the UE, which forces node B to search over its entire cell, and to the typically low power of node B's received UL signal and the low number of UL DPCCH pilot symbols, which forces node B to receive a large number of symbols before reliable channel and path estimates are obtained.
In order to reduce this time delay, which contributes to the period during which TPC on the new UL and DL is open-loop, the physical layer (Layer 1) in node B gets Layer-3 information from the RNC to start UL synchronization before the Layer-3 “Active Set Update” message is transmitted to the UE. Although the amount of improvement due to this for node B is not easily calculated, there has been an indication in at least one RNC log of a delay of only 30-40 ms. The UL (node B) has at least two other timing advantages over the DL in establishing sync: the Active Set Update message is itself 20 ms long, and then the UE needs time to process it. The UE's processing time depends on the UE's architecture and on the current load on the real-time processing units in the terminal. A further delay of 30-50 ms might occur in a terminal before the terminal starts to combine the new DL information on Layer 1. The sum of these delays in the DL is about 100 ms, which means that the start of UL synchronization can be expected to occur at least about 100 ms before DL synchronization (sync) occurs. The end of UL sync, however, can occur after the UE has received the active set update message and started to combine power control commands from the new base station. In this situation, there is a risk of control loop problems, in the form of UL power peaks, i.e., too large UL power, or UL power dips, i.e., too small UL power.
Field experiments have shown a phenomenon during soft handovers that is apparently not prevented by current TPC methods. When a terminal or user equipment enters or adds a communication link in a soft handover, peaks of 20-40 dB in the uplink (UE-to-base) transmitted power can be observed if the initial downlink power on the new link is set too high and if the new node B fails to achieve uplink synchronization within 30-40 ms from the time the UE starts to combine downlink TPC commands (i.e., after receiving and processing the “Active Set Update” message). It has also been observed in field trials that there is some risk that a communication link established in SHO goes Out of Sync (OoS) after the link has been in sync. If this happens, the link would usually be resynchronized in a procedure that is the same as the procedure in the initial sync case, and hence there will be a risk for large UL transmit power peaks.
TPC on the UL and DL of a new or resynchronized connection in a soft handover may operate open-loop for 100-200 ms due to the time needed for UL synchronization, and such lengthy delays appear to be the main cause of the peaks in the transmitted power. These power peaks are interference to other users, and thus can cause problems for the users and the system as a whole. UL sync detection methods and apparatus are described in U.S. patent application Ser. No. 10/839,926 filed on May 6, 2004, by B. Lindoff et al. for “Synchronization Detection Methods and Apparatus”.